Amy - crisps are a good lightweight filler for packages of America. Flavours don't really exist over there and Salt & Vinegar is much missed by all Brits I know, and loved by US friends too. I always send S&V hula hoops and they go down a treat. Pombears also very popular. Chicken and Prawn cocktail crisps seem to induce more bemusement than love! (I will never forget the look on my daughter's teacher's face when they offered her a skip at recess one day! Horrified is not the word!)
Chocolate advent calendars also not a thing in the US so I have just sent a couple of those this week along with some selection boxes.
Our American friends all ask for fruitella juice chews, jelly babies and millions - there's nothing like them over there and they all love them.
In terms of non-food stuff my kids clothes from the likes of M&S and Next were always commented on really positively when we lived there and I have since started sending nice t-shirts etc to friends - just classier, less tacky styles than what was generally available locally to us in TX. Also pure cotton clothes were few and far between so cotton PJs from M&S were a treat for us.
There is surprisingly little crossover with kids books so you could easily choose some books to send which would be new to them.